Could this constraint apply in other ways? Suppose magic is the result of something that responds to the wishes of witches, as suggested at one point. If that something is Earth-based, perhaps a wizard on an outbound spacecraft would stop being able to do magic when he reaches 6 light-hours out. An interesting experiment.

Harry might be able to realistically do an experiment similar to this as a first year if there is a magic spell that lets you communicate with an object. He could use a spell to accelerate that object to a very high speed and then check in on it as it approaches the 6 light-hour point.